1. Field of the Invention
The present invention relates to a dimmable LED (light emitting diode) driving circuit, especially to a phase cut dimmable switching mode LED driving circuit without phase angle measurement.
2. Description of the Related Art
Conventional dimmable lighting circuits use a triac-based dimmer to perform a phase-cutting operation on a standard AC (alternating current) voltage source (60 Hz/110 Vin North America or 50 Hz/220 V in Europe) in response to a dimming operation, so as to control the input power and thereby control the lighting condition of a traditional lamp.
Due to a longer lifespan and a better electrical efficiency, LED lamps have been gradually replacing traditional lamps. In applications where a traditional lamp is replaced with an LED lamp, apart from maintaining a good power factor, the driving circuit for the LED lamp is generally required to provide a constant output current to ensure a stable lighting condition. To meet these requirements, a switching mode LED driving circuit is generally designed to have a PFC (power factor correction) function and a constant output current control mechanism. That is, the switching mode LED driving circuit for driving the LED lamp is generally designed to make an envelope of an input switching current in phase with an input voltage and in the meanwhile provide a constant output current to an LED lamp. However, when a dimmer circuit for providing a dimming function is placed between a standard AC voltage and the generally designed switching mode LED driving circuit, the operation of the dimmer circuit can be ineffective in dimming the LED lamp. The reason is that when a phase-cut AC voltage is applied to the generally designed switching mode LED driving circuit for adjusting the lighting condition, as the output current will remain unchanged, the lighting condition will also remain unchanged, and the dimming function will not be achievable. Besides, as the phase-cut AC voltage has a less conduction time than a standard AC voltage, to make a same constant output current, the peaks of the input switching current caused by the phase-cut AC voltage will be higher than the peaks of the input switching current caused by the standard AC voltage, and the higher peaks of the input switching current can cause damage to the components of the driving circuit.
Please refer to FIG. 1a-1c, which illustrate the waveforms of the input switching current caused by three different phase-cut AC voltages in comparison with the waveform of the input switching current caused by a standard AC voltage under a constant on-time and constant output current control of a switching mode LED driving circuit of prior art. As is known in the art, the constant on-time control can result in a good power factor and a constant output current by utilizing a constant on-time generation mechanism, in which the on time per switching cycle has a same duration when in a stable state, and the length of the same duration is dependent on the level of an average of an amplified error signal, the amplified error signal being generated according to a difference between a reference voltage and the voltage of a sensed signal of the input switching current.
When under different operation conditions, the length of the same duration of the on time per switching cycle will be automatically adjusted to required values to maintain a constant output current. For example, when the AC voltage gets lower or the load increases, the length of the same duration of the on time per switching cycle will be prolonged, and when the AC voltage gets higher or the load decreases, the length of the same duration of the on time per switching cycle will be shortened. Besides, the constant on-time control also implies a variable-frequency operation, in which the off-time per switching cycle will be reduced to increase a duty cycle when an input voltage decreases, and prolonged to decrease a duty cycle when the input voltage increases. As shown in FIG. 1a-1c, due to the constant on-time control the envelope of an input switching current takes the form of different phase-cut sinusoidal waveforms in response to different phase-cut AC voltages so as to provide a good power factor, and the off-time is variable and its length is dependent on the level of an input voltage—the higher the input voltage is, the longer the off-time will be. Besides, different constant on times Ton2, Ton3, and Ton4 are generated in response to different phase-cut AC voltages compared with a constant on time Ton1 generated in response to a standard AC voltage so as to maintain a same constant output current, wherein Ton4>Ton3>Ton2>Ton1. However, with the level of the constant output current remains unchanged, the dimming function will not be achievable.
To solve the problem, some applications of prior art have managed to adjust a reference current level according to a phase angle of the conduction periods. Please refer to FIG. 2, which illustrates a circuit diagram of a dimmable switching mode LED driving circuit of prior art. As illustrated in FIG. 2, the dimmable switching mode LED driving circuit includes a controller 10, a triac-based dimmer 20, a bridge rectifier 30, a switching power converter 40, an LED load 50, a power switch 60, and a current sensing resistor 70. As illustrated in FIG. 2, the controller 10 includes a dimmer detector 11, a reference current adjusting unit 12, and a switching power converter current controller 13, so as to adjust a reference current level according to a phase angle of the conduction periods of an AC voltage VAC, and thereby provide a dimming function.
However, as a TRIAC voltage (VTRC) of the triac-based dimmer 20 tends to be noisy, the phase angle detection circuit of the dimmable switching mode LED driving circuit of FIG. 2 has to be carefully designed. Besides, using the phase angle as a variable in determining the reference current level will make the entire control circuit much more complex.
To solve the foregoing problem, a novel dimmable switching mode LED driving circuit is needed.